Television transmission system



Aug. 8, 1944. A ROSENTHAL 2,355,110

TELEVIS ION TRANSMISS ION SYSTEM Filed Feb. 23, 1939 Patented Aug. 8, 1944 UNITED STATES PATENT OFFICE.

TELEVISION TRANSMISSION SYSTEM Adolph Henry Rosenthal, New York, N. Y., as-

slgnor, by mesne assignments, to Scophony Corporation of America, New York, N. Y., a

corporation of Delaware Application February 23, 1939, Serial No. 258,069 In Great Britain February 25, 1938 17 Claims.

number of small electric condensers arrangedover the signal plate or screen, and these charges stored during the picture'period are released by scanning the screen. Thus the luminous energy of the image is converted into electrical energy which is stored by the condensers, and is released by the scanning beam.

The present invention is concerned with the production of a novel form of image screen. By the term. image screen" is meant a screen adapted to transform the luminous energy of the object to be transmitted into an electrical form of energy, to store this energy and to release it when scanned. To this end use is made of certain physical efiects, discovered and investigated in connection with scientific work on the electric phenomena in luminous phosphors and certain crystals, these effects being also closely connected with the conduction of/electricity in solids and with electrical phenomena connected with the formation, storage and extinction of the latent photographic image.

This work has revealed that when certain crystals (particularly alkali halide crystals), and phosphors are irradiated with an exciting radiation such as light of a suitable wavelength, or cathode rays, certain sensitive "centres in these substances are transformed from an initial state of lower energy to an excited state of higher energy. This higher state possesses considerable stability, but the stored-up energy can be released, i. e., the "centres in the excited state can be brought back to the initial state by certain external influences. This release of the stored-up energy can be effected by influencing the substance with light of a suitable wavelength (usually difierent from that of the exciting light and in the red or infra-red spectral regions), with other radiations such as cathode rays, with heat or with electric or magnetic fields. ing the stored-up energy will be hereinafter referred to as quenching and a radiation effecting this will be termed a quenching radiation.

In the case of certain substances such as many luminous phosphors, the quenching is accompanied by a sudden emission of light, 1. e., a considerable part of the stored-up light energy is released again as light during the quenching process. But in general, whether such luminescence occurs or not, the quenching as well as the excita- The action of releasr tion is usually connected with the liberation and the transportation of electric charges in the interior of the substance. It is assumed to-day that during the excitation, electrons are liberated from certain sensitive centres in the substance and are loosely bound in higher semi-stable energy levels from which they are freed again by the quenching process. Also without a quenching process a certain number of the excited "centres fall back to the initial energy state, this process being mainly caused by thermal collisions with neighbouring atoms, and increasing with rising temperature of the substance.

In the alkali halide crystals centres of the above mentioned kind are known as Farbzentren" or "colour centres and U-centres and they can be artificially produced in the crystal by various methods. For example the colour centres can be produced by heating a crystal in the vapour of its own alkali metal or by bombarding the crystal with cathode rays. A crystal containing such colour centres possesses a different absorption range for a luminous exciting radiation than a crystal without such colour centres. In general, the luminous exciting radiation must be in the ultra-violet spectral range for a crystal without colour centres, but by providing the crystal with colour centresthe spectral range of the luminous exciting radiation can be shifted into the visible portion of the spectrum. I

If such material is placed in an electric field the'poles of which are connected by an external circuit and the substance is illuminated by an exciting radiation, then a small current will flow in the circuit. If a quenching radiation is applied to the substance the current will suddenly increase in value and will then return to a very low value. For a given temperature the increase in the current depends upon the intensities of the exciting and quenching radiations. The higher the temperature the smaller will 'be the sudden increase in the current.

According to the present invention a television transmitter comprises an image screen of a material of the type described, means for subjecting said image screen to the influence of exciting and quenching radiations, said means comprising image forming means for utilising one of said radiatons to form an image of the object to be transmitted on said image screen, and focussing and deflecting means for utilising the other of said radiations to form a scanning beam, a signal plate associated with said image screen and serving as one electrode of an electric field in which said image screen is situated, the image-forming radiation falling on each volume element of said image screen causing the energy contained in said element to assume a level which difiers from a fixed datum level by an amount depending on the intensity of the radiation and the scanning radiation causing said energy to return to said datum level, said return being accompanied by a flow of current of a magnitude dependent upon the intensity of the image-forming radiation in an external circuit connected to said signal plate.

According to one form of the invention the image-forming radiation acts as the quenching radiation and the scanning radiation serves as the exciting radiation, in which case the fixed datum level of energy is high and corresponds to the excited state of higher energy previously mentioned.

According to an alternative form of the invention the image-forming radiation acts as the exciting radiation and the scanning radiation serves as the quenching radiation, in which case the fixed datum level of energy is low and corresponds to the initial state of lower energy previously mentioned.

In choosing the appropriate radiations the following practical considerations must be borne in mind. If light is employed both as, the imageiorming and scanning radiations, then the intensity of the latter must be high relative to that of the former. If an electron image of the object is formed on the image screen, then an electron beam should be employed for scanning. Ii light is employed as an exciting radiation (either as the image-forming or as the scanning radiation) then the absorption range of the image screen should be in the visible portion of the spectrum In the case of an alkali halide crystal this can be achieved, as mentioned above, by providing the crystal with colour centres. The spectral range of light used as a quenching radiation (either as the image-forming or as the scanning radiation) will depend upon the material of the image screen. In the case of certain alkali halide crystals, such as potassium chloride normal white light can be used, although usually it is preferable that the light should contain a substantial red component. In any case the addition of the sensitizers hereinafter described to the material will enable light of any desired spectral range to be used.

The term crystal as applied to any of the above materials also includes a micro-crystalline structure.

The invention will now be described by way of example with reference to the accompanying drawing in which;

Fig. 1 shows schematically a television transmitter according to the invention in which a luminous image is formed on the image screen and a cathode ray beam is used for the scanning;

Figs. 2 and 3 are diagrams illustrating two alternative methods of operating the arrangement of Fi 1;

Fig. 4 shows schematically a transmitter in which an electron image of the object is formed on the image screen; and

Fig. 5 shows the use of a light beam for scan ning the image screen.

Referring to Fig. 1, a transmitter tube i comprises an image screen consisting of an alkali halide crystal I mounted on a signal plate 3. A luminous image of the object 4 is formed on the surface of the crystal 2 by means of the lens 5. A beam 6 of cathode rays proceeding from the cathode I is deflected by the pairs of coils 8, 8 to scan the surface of the crystal 2. A metal coating l0 inside the tube serves as an anode, and also as the negative electrode of an electric field, in which the crystal is situated, the signal plate I serving as the positive electrode of this field. The

output circuit of the device includes an impedance l I, across which the picture signals are developed.

The luminous energy of the image serves as an exciting radiation, and to this end the crystal is previously prepared to produce colour centres therein, either by heating the crystal in the vapour of its own alkali metal, or by bombarding the crystal with cathode rays. The cathode ray beam 6 serves as the quenching radiation, and the appropriate quenching action can be regulated by adjusting the potential on the control electrode I! to adjust the strength of the beam.

Each element of the luminous image 01 the object projected on to the crystal 2 will excite a corresponding elemental volume oi the crystal to a more or less'degree depending upon the intensity oi! the element. There is thus stored in each corresponding volume element of the crystal 9. corresponding amount of energy for the whole time existing between two successive scannings ot a given elemental area of the crystal. At the instant when such an area is scanned with the beam 8 of quenching radiation most of this energy is freed in the form of temporarily free electrons within the volume element of the crystal. These free electrons move toward the signal plate 3, thus producing a momentary increase in the constant current in the external circuit, this increase being proportional to the intensity oi'the exciting and quenching radiations falling on the elemental area of the crystal.

Thus there will flow in the impedance II a direct current component which is the sum of all the constant currents flowing through the individual volume elements, and the intensity of which for a given temperature depends upon the average intensity of the whole image projected on the crystal. Superimposed on this constant current will be the current impulses produced by the scanning beam 6. Hence a varying voltage will be produced across the impedance Ii which can be used to modulate the amplitude of a carrier wave, the mean amplitude of which can be determined by the direct current component, and from which the mean picture intensity at a receiver can be derived.

To obtain an eiiicient transformation of the luminous intensity of the image elements into voltage impulses it is necessary that the magnitude of the current changes should be large relative to the magnitude of the constant direct current in one elemental volume of the crystal. To achieve this the intensity of the quenching beam 6 should be intense, and the temperature of the image screen should be maintained at a moderate value, for, as mentioned above, with increasing temperature, the centres of the crystal tend to return from the higher energy state to the initial energy state, independently of the quenching radiation, with a resulting loss in the stored energy and an increase in the direct current component.

The variation in the energy contained in a volume element of the material with time is illustrated in the curve of Fig. 2 in which the ordinates represent the energy, and the abscissae represent time. During the period 11-11 the volume element is illuminated with the luminous exciting radialevel e during the time b-c which represents one picture element duration. The current in the impedance I I will be represented by a curve which is the first difl'erential of the curve of Fig. 2.

The apparatus of Fig. 1 can be modified so that the luminous energy of the image serves as the quenching radiation, and the cathode ray beam serves as the exciting radiation. Since there is not a great deal of freedom in choosing the spectral composition of the light which forms the image on the crystal, the latter must be suited to the nature of the light, 1. e., it must exhibit an absorption range for a luminous quenching radiation which corresponds to the spectral range of the light. For normal white. light a potassium chloride crystal sensitized with thallium chloride is suitable. The intensity of the beam 8 must be regulated to provide a suitable exciting action; in

I general, its intensity must be less than when used as a quenching radiation.

The operation of the device in this case is illustrated in Fig. 3. During the period -17 (the frame period) the luminous quenching radiation of the image reduces the energy of a volume element of the crystal from a high datum level e1 to a low level e depending upon the intensity of the radiation. During the period b-c (the duration of a picture element) the exciting beam 6 suddenly returns the energy to the high level e1. Again the current in the impedance II will be represented by a curve which is the first differential of the curve of Fig. 3.

In order to keep the temperature of the crystal constant, simple thermostatic means may be provided. For example the portion of the tube I surrounding the crystal may be enclosed in an electrical oven, the heating current of which is controlled by a thermo-couple inserted in the crystal itself.

An alternative form of apparatus is shown in Fig. 4. An optical image of the object 4 is formed on the semi-transparent photo electric layer l3 of the tube by means of the lens]. The electrons emitted by the layer I 3 are focussed on the crystal 2 by means of the electron optical system IO, M, l5, to form on the surface of the crystal an electron-optical image of the object 4. .The crystal is scanned with the cathode ray beam 6 and picture signals are developed across the impedance II. The intensity of the electron optical image is such as to excite the volume elements of the crystal, while the higher intensity of the cathode ray beam 8 permits this beam to serve as the quenching radiation. The operation of the apparatus is the same as that described with reference to Figs. 1

and 2.

In Fig. light is employed both for the imageforming and scanning radiations. The crystal 2 is provided on each face with a semi-transparent electrode l6, II, which can be a metal film or an electrically conducting oxide such as zinc oxide. The lens 5 forms an image of the object 4 on one side of the crystal, and the opposite side is scanned with a beam of light ll, which is produced by an optical system (shown diagrammatically at If!) from the are 20 and which has the scanning motion imparted thereto by the scanning members 2|, 22. The electrodes It, I! are connected in series with a source of potential 23 and the impedance H, across which the picture signals are developed.

The crystal 2 is provided with colour centres so that the light from the object will exert the necessary exciting action. The scanning light, which is of a very high intensity relative to that of the image-forming light serves as the quenching radiation, and it is preferably such that a considerable part of its energy is concentrated in the red or infra-red portion of the spectral tensity relative to the image-forming light..

Many modifications of the arrangement shown in Figs. 1 and 4 are possible. For example, the necessary electric field for the crystal 2 can be produced by a pair of electrodes as described in connection with Fig. 5, one of which serves as the signal plate. Also, the crystal 2 can be situated outside the tube l, in which case it is'separated from the interior thereof by a thin layer of metal foil permeable to electrons, which layer can serve as one of the electrodes.

The surface of the crystal which receives the exciting and quenching radiations can be provided with a coating of a material having'a high ratio of secondary electrons emitted to primary electrons incident thereon, whereby an increase in the effect can be obtained. When using an alkali halide as the image screen, thi need not be in the form of a single crystal as shown. For example it can be in the form of a micro-crystalline layer which is preferably produced by evaporating onto the signal plate or other carrier, a. powder of the material carried in a small metal boat or container inside the tube and heated by eddy currents induced therein from an external coil.

In all cases the spectral range of absorptionof the substance to the quenching and also to the exciting radiation can be shifted if desired to suitable spectral regions by adding to the substance small traces, of sensitizers in the form of suitable heavy metals such as silver or thallium. This can be effected during the manufacture by adding the desired sensitizers to the substance in the molten state. In the case of alkali halides the sensitisation can also be effected in other ways, for example by building into the crystal lattice, compounds with foreign molecules such as hydrogen, oxygen or carbon monoxide. This can be done by heating the crystal in the atmosphere of one of these gases. Suitable colour filters may also be used in addition, if desired, to correct the spectral ranges of the various absorption spectra.

The use of an image screen according to the present invention possesses important advantages over the use of the usual photo-electric or secondary electron emitting screens in that volume eflects instead of surface effects are employed. When a surface effect is employed, the interchange or redistribution efiects of elctrons on the surface between neighbouring elemental areas of different intensity reduce the efliciency and partly annul the advantages gained by the storage effect, besides giving rise to so-called spurious signals superimposed on the image si nals. In the present case such surface redistribution effects are absent or are so small that they duce the statistical current fluctuations producdiations to form an image oi the object to be transmitted on said image screen and being adapted to maintain the intensity of the imageforming radiation within that range of intensities for which the electrical energy contained'in any volume" element of said image screen is caused, by said image forming radiation, to differ from a fixed datum level by an amount depending on the intensity of the radiation, said first mentioned means urther comprising i'ocussing and deflecting me for. utilizing the other of said radiations to) form a scanning beam, and

means for maintaining the intensity of said other radiation within that range of intensities for which the electrical energy contained in any vol-- ume element of said image screen is caused, when struck by said scanning beam, to return to said datum level, and a signal plate associated with said image screen and serving as one electrode of an electric field in whichsaid image screen is situated, said signal plate being adapted to be connected to an external circuit so that said return will be accompanied by a flow of current in said circuit.

2. In a television transmitter an image screen of a material of the type in which the internal energy can be raised to a high level bythe influence of an exciting radiation and returned to a low level by the action of a quenching radiation, means for utilizing an exciting radiation to form an image of the object to be transmitted on said image screen and being adapted to maintain said exciting radiation within that range of intensities for which the electrical energy contained in any volume element of said image screen is increased, by said exciting radiation, by an amount depending upon the intensity of the radiation, focussing and deflecting means for utilizing a quenching radiation to form a scanning beam, means for maintaining the intensity of said quenching radiation within that range of.

intensities for which the electrical energy contained in any volume element of said image screen is caused, when struck by said scanning beam, to return to a datum level, and a signal plate associated with said image screen and serving as one electrode of an electric field in which said image screen is situated, said signal plate being adapted to be connected to an exexternal circuit so that said return will be accompanied by a fiow of current in said circuit.

3. In a television transmitter an image screen of a material of the type in which the internal energy can be raised to a high level by the infiuence of an exciting radiation and returned to a low level by the action of a quenching radiation, means for utilizing a quenching radiation to form an image of the object to be transmitted on said screen and being adapted to maintain said quenching radiation within that range of intensities for which the electrical energy contained in any volume element of said image screen is reduced, by said quenching radiation, by 18 an amount depending upon the intensity of the radiation, iocussing and deflecting means for utilizing an exciting radiation to form a scanning beam, means for maintaining the intensity of said exciting radiaton within that range of intensities for which the electrical energy contained in any volume element or said image screen is caused, when struck by said scanning beam, to rise to a datum level, and a signal plate associated with said image screen and serving as one electrode of an electric field in which said image screen is situated, said signal plate being adapted to be connected to an external circuit so that said energy rise will be accompanied by a flow of current in said circuit.

4. In a television transmitter an image screen of 'a material of the type in which the internal energy can be raised to a high level by the influence of an electromagnetic exciting radiation lying in the visible range of wavelengths and returned to a low level by the action of aquenching radiation, means for subjecting said screen to the influence of an electromagnetic exciting radiation lying in the visible range of wavelengths and means for subjecting said screen to the influence of a quenching radiation, said means comprising means for utilizing one of said radiations to form an image of the object to be transmitted on the image screen and being adapted to maintain the intensity of the image forming radiation within that range or intensities for which the electrical energy contained in any volume element of said image screen is caused, by said image forming radiation, to diiler from a fixed datum level by an amount depending on the intensity of the radiation, said means for subjecting said screen tothe influence of radiations further comprising focussing and defleeting means for utilizing the other of said radiations to form a scanning beam, and means for maintaining the intensity of said other radiation within that range of intensities for which the electrical energy contained in any volume element of said image screen is caused, when struck by said scanning beam, to return to said datum level, and a signal plate associated with said image screen and serving as one electrode of an electric field in which said image screen is sit- -uated, said signal plate being adapted to be connected to an external circuit so that said return will be accompanied by a flow of current in said circuit.

5. In a television transmitter an image screen consisting of an alkali halide material of the type in which the internal energy can be raised to a high level by the influence of an exciting radiation and returned to a low level by the action of a quenching radiation, means for subjecting said screen -to the influence of radiations adapted to act as exciting and quenching radiations in respect of said material, said means comprising means for utilising one of said radiations to form an image of the object to be transmitted on said image screen and being adapted to maintain the intensity of theimage-forming radiation within that range of intensities for which the'electrical energy contained in any volume element of said image screen is caused by said image forming radiation, to differ from a fixed datum level by an amount depending on the intensity of the radiation, said first mentioned means further comprising focussing and deflecting means for utilising the other of said radiations to form a scanning beam and means for maintaining the intensity of said other radiation withinthat range of intensities for which the electrical energy contained in any volume element of said image screen is caused, when struck by said scamiing beam, to return to said datum level, and a signal plate associated with said image screen and serving as one electrode of an electric field in which said image screen is situated,

said signal plate being adapted to be connected to an external circuit so that said return will be accompanied by a flow of current in said circuit.

6. A transmitter incorporating the invention set forth in claim 5, wherein light is employed as said quenching radiation and serves as said image-forming radiation and a cathode ray beam serves as said exciting radiation.

7. In a television transmitter an image screen consisting of an alkali halide material adapted to have its internal energy raised to a high level by the influence of an electromagnetic exciting radiation lying in the visible range of wavelengths and to have said energy returned to a low level by the action of a quenching radiation, means for subjecting said screen to the influence of an electromagnetic exciting radiation lying in the visible range of wavelengths and means for subjecting said screen to the influence of a quenching radiation, said means comprising means for utilising one of said radiations to form an image of the object to be transmitted on said image screen, and being adapted to maintain the intensity of the image-forming radiation within that range of intensities for which the electrical energy contained in any volume element of said image screen is caused, by said image forming radiation, to differ from a fixed datum level by an amount depending on the in tensity of the radiation, said means for subjecting said screen to the influence of radiations further comprising forcussing and deflecting means for utilising the other of said radiations to form a scanning beam and means for maintaining the intensity of said other radiation within that range of intensities for which the electrical energy contained in any volume element of said image screen is caused, when struck by said scanning beam, to return to said datum level, a signal plate associated with said image screen and serving as one electrode of an electric field in which said image screen is situated, and an external signal circuit connected to said signal Plate.

8. In a television transmitter an image screen consisting of a material of the type in which the internal energy can be raised to a high level by the influence of an exciting radiation and returned to a low level by the action of a quenching radiation, means for subjecting said screen to the influence of exciting and quenching light ra-' diations said means comprising means for utilising one of said light radiations to form an image of the object to be transmitted on said image screen and being adapted to maintain the intensity of the image forming radiation within that range of intensities for which the electrical energy contained in any volume element or said image screen is caused, by said image forming radiation to differ from a fixed datum level by an amount depending on the intensity of the radiation, said first mentioned means further comprising light focussing and deflecting means for utilising the other of said light radiations to form .a scanning beam, and means for maintaining the intensity of said other light radiation within that range of intensities for which the electrical energy contained in any volume element of said screen is caused, when struck by said scanning beam, to return to said datum level, and a signal plate associated with said image screen and serving as one electrode of an electric field in which said image screen is situated, said signal plate being adapted to be connected to an external circuit so that said return will be accompanied by a flow of current in said circuit.

9. In a television transmitter an image screen consisting of a material of the type in which the internal energy can be raised to a high level by the influence of an exciting radiation and returned to a low level by the action of a quenching radiation, means for subjecting said screen to the influence of exciting and quenching electron radiations, said means comprising means for utilising one of said radiations to form an electron image from an optical image of the object to be transmitted on said image screen and being adapted to maintain the intensity of the imageforming electron rediation within that range of intensities for which the electrical energy contained in any volume element of said screen is caused, by said image-forming lectron radiation, to different from a fixed datum level by an amount depending on the intensity of the radiation, said first mentioned means further comprising electron-optical forcussing and electron deflecting means for utilising the other of said electron radiations to form a scanning beam, and means for maintaining the intensity of said other electron radiation within that range of intensities for which the electrical energy contained in any volume element of said image screen is caused, when struck by said scanning beam, to return to said datum level, and a signal plate associated with said image screen and serving as one electrode of an electric'field in which said image screen is situated, said signal plate being adapted to be connected to an external circuit so that said return will be accompanied by a fiow of current in said circuit.

10. The invention set forth in claim 1, wherein the material of said screen is provided with small traces of a sensitizing agency.

11. The invention set forth in claim 1, wherein the material of said screen is provided with small traces of a heavy metal acting as a sensitizer.

12. The invention set forth in claim 1, wherein the material of said screen-is provided with colour centres.

13. The invention set forth in claim 1, wherein said quenching radiation is electro-magnetic and lies in the visible range of wave lengths.

14. The invention set forth in claim 1, wherein said image screen material is an alkali halide and said quenching radiation is electro-magnetic and lies in the visible range of wave lengths.

15. The invention set forth in claim 1, wherein said image screen material is of micro-crystalline form.

16. The invention set forth in claim 1, wherein said image screen includes a substance having a high degree of secondary electron emission.

17. The invention set forth in claim 1, including means to maintain said image screen material at a predetermined temperature.

' ADOLPH HENRY ROSENTHAL. 

